1. Field of the Invention
This invention relates to preserving the electrical and optical properties of optically transparent and conductive films such as indium tin oxide (ITO), and more particularly, to providing a buffer or protective layer between aluminum and ITO for use in the fabrication of flat panel displays and the like.
2. Description of the Related Art
Optically transparent and electrically conductive materials such as indium tin oxide (ITO) find utility in flat panel display (FPD) industries such as field emission displays (FEDs), liquid crystal displays (LCDs), and organic light emitting devices (OLEDs), as well as in solar cells. Surface and bulk characteristics are imperative to the quality of electrical and optical properties of these and other optically transparent and electrically conductive films. It is therefore very important to ensure that such films exhibit the desired surface and bulk properties such that the desired degree of transmission of visible light and electrical properties are obtained.
Devices incorporating ITO often use an aluminum layer deposited over the ITO. For instance, in an FED device where the faceplate is connected to the baseplate using spacers, aluminum is often deposited over the ITO layer in the faceplate to establish sites for the bonding of misaligned spacers. More particularly, an aluminum layer is formed over the ITO layer, the aluminum layer having wells extending therein to the surface of the ITO layer. Bond pads are provided within these wells against the ITO layer at the desired spacer locations. Then, when an array of spacers is brought against the faceplate for anodic bonding, desired spacers contained in the array will bond to the bond pads, while other, misaligned spacers will bond to the aluminum layer. After bonding is complete, the aluminum layer with the misaligned spacers bonded thereto can be removed to leave the desired spacer configuration in the FED.
A problem with using aluminum with ITO in the above and other applications is that ITO is susceptible to corrosion in the presence of aluminum. Atomic and/or ionic diffusion occurs through the aluminum to the ITO during processes such as anodic bonding, thermal cycling, thermal diffusion processes, low energy ion implantation processes, and processes which include electric and/or magnetic fields. ITO is especially susceptible to corrosion in the presence of aluminum when exposed to alkaline or basic solutions or solvents. See J. E. A. M. van den Meerakker and W. R. ter Veen, J. Electrochem. Soc., vol. 139, no. 2, 385 (1992). Corrosion of ITO in alkaline solutions produces SnO32xe2x88x92, which dissolves in the solution, and In metal, which forms grains at the surface. This causes a gray opaque appearance and a disconnection between the ITO and aluminum. Corrosion of the ITO can prove fatal in devices such as flat panel displays by reducing or eliminating the electrical conductivity and optical transparency of the ITO material. This corrosion can also cause delamination of the aluminum layer from the ITO. Redeposition of corrosion byproducts onto the substrate leads to additional defects, e.g., particle defects.
Furthermore, during anodic bonding of spacers to bond pads, excess oxide can change local optical properties of the adjacent ITO between the bond pads. Optical properties may also be changed due to etching.
Accordingly, what is needed is an improved method and apparatus for protecting the electrical and optical properties of an ITO layer and the like when such a layer is exposed to aluminum.
Briefly stated, the needs addressed above are solved by providing an aluminum oxide layer between an aluminum layer and an ITO layer to protect the ITO from optical and electrical defects sustained, for instance, during anodic bonding and other fabrication steps. This aluminum oxide barrier layer is preferably formed either by: (1) partially or completely anodizing an aluminum layer formed over the ITO layer, or (2) an in situ process forming aluminum oxide either over the ITO layer or over an aluminum layer formed on the ITO layer. After either of these processes, an aluminum layer is then formed over the aluminum oxide layer.
In accordance with one aspect of the present invention, a method of manufacturing a tin oxide/aluminum structure is provided. The method comprises forming a tin oxide layer, forming an aluminum oxide layer over the tin oxide layer, and forming a top aluminum layer over the aluminum oxide layer. In one embodiment, the aluminum oxide layer is formed by anodizing aluminum. In another embodiment, the aluminum oxide layer is formed by reactive sputtering.
In accordance with another aspect of the present invention, a tin oxide/aluminum structure is provided comprising a tin oxide layer over a substrate, an aluminum oxide layer over the tin oxide layer, and an aluminum layer over the aluminum oxide layer. In one embodiment, the tin oxide layer comprises indium tin oxide. A second aluminum layer may be provided between the tin oxide layer and the aluminum layer.
In accordance with another aspect of the present invention, a method of protecting an indium tin oxide layer in the presence of aluminum is provided. An aluminum oxide layer is formed between the indium tin oxide layer and the aluminum. The aluminum oxide layer is preferably formed either by anodizing the aluminum or by reactive sputtering.
In accordance with another aspect of the present invention, a method of fabricating a display device structure is provided. The method comprises forming an indium tin oxide layer, forming an aluminum oxide layer over the tin oxide layer, and forming an aluminum layer over the aluminum oxide layer. The structure is then exposed to an indium tin oxide-corrosive medium, such as would be used during the fabrication of the display device. The aluminum oxide prevents diffusion of the corrosive medium through the aluminum layer to the indium tin oxide layer. Once the structure is no longer exposed to the indium tin oxide-corrosive medium, the aluminum oxide and aluminum layers are removed. In one embodiment, these layers are removed after spacers have been fabricated. More preferably, by using an aluminum oxide barrier layer between the indium tin oxide layer and the aluminum layer, the aluminum oxide and aluminum layers can be removed using an etchant comprising phosphoric acid at a temperature up to about 60xc2x0 C., without damaging the indium tin oxide.
In accordance with another aspect of the present invention, a display device structure comprises a substrate, an electrically conductive and optically transparent layer over the substrate, an aluminum oxide layer over the electrically conductive and optically transparent layer, and an aluminum layer over the aluminum oxide layer. In one embodiment, the aluminum oxide layer has a thickness of between about 500 and 1,500 xc3x85, and the aluminum layer has a thickness of between about 4,500 and 6,000 xc3x85. The aluminum oxide layer preferably comprises AlOx where x is between about 0.25 and 1.5.